I have a 115V/230V transformer with specifications of: outputs 10V (parallel)/20V (series) and 1.2A (parallel)/600mA (series). Product

I'm no electrical engineer, but I need to get this set up to power my first hobbyist project. The reason for purchasing this particular transformer was due to height restrictions.

From what I understand, I can set this transformer up with 120V in parallel by connecting pins 1/3 to LOAD and 2/4 to NEUTRAL. If I then hook up pins 5/6 and 7/8 in parallel, I will get 10V/1.2A output.

I have a cheap oscilloscope that I am using to measure the transformer output. I hooked it up to the test leads (2Vp-p 1kHz square wave) and it looks like it's working. Test wave


simulate this circuit – Schematic created using CircuitLab

When I hook up to the transformer output, I am not getting the expected A/C sine wave. Instead, it looks like the sine wave is cut off at the 5V mark. The measurements are 10.3Vp-p, 4.81 Vrms 60.02 Hz AC wave

Is my oscilloscope incorrect or is this the correct output for a 10V parallel transformer? I was expecting a 10 Vrms output (with peaks up to 14.14 V)

I also have a similar 9V AC transformer (single primary single secondary) that produces a similar output. Are my assumptions incorrect about these transformers?

  • \$\begingroup\$ C1 has no discharge path, so it is at its maximum voltage ... that prevents current from flowing in the circuit as it would if the circuit was powering a load \$\endgroup\$ – jsotola Sep 21 '19 at 21:33
  • \$\begingroup\$ Looks like your oscilloscope input is saturating. Try a x10 probe (and change the software setting to get it to read right, but that's not as critical). \$\endgroup\$ – Spehro Pefhany Sep 21 '19 at 22:35
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    \$\begingroup\$ @SpehroPefhany I suggest you make this an answer, cause it looks like you are right. The input of my oscilloscope says Max 35V, but I assumed that was at x1 probe. Switching to x10 probe and x10 in the software, it's registering a perfect sine wave. Good to know for future projects, thanks! \$\endgroup\$ – Jack Sep 22 '19 at 14:00

Looks like your oscilloscope input is saturating. The input will have a certain range and beyond that you need to add attenuation (as in a x10, x100 etc. probe) or you will get that effect (and can possibly damage the oscilloscope at some point, though most should be protected to a couple hundred volts peak or so).

Try a x10 probe (and change the software setting to get it to read right, but that's not as critical).

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    \$\begingroup\$ +1. Annoyed to have missed that :-). \$\endgroup\$ – Russell McMahon Sep 23 '19 at 0:06

Ensure that your "dottings" are correct.
If your primary one are wrong you'd probably get magic smoke.
If the secondaries are wrong it may be "interesting".
You should get nothing but may get some low output, such as you are seeing.

When the secondaries are operated individually do they produce the expected voltage.

Join any two matching ends of the secondaries and observe the voltage at the two open ends. If they are properly matched in polarity you'd expect zero volts.
If of opposite polarity / dotting you'd expect 2 x Vsecondary.

  • \$\begingroup\$ According to the datasheet, I believe the dots are correct. Pins 1/3 are dotted, 2/4 are not. However the pins 3/4 are reversed on the casing, so they go 1, 2, 4, 3. I have pin 2 and pin 4 jumped and pin 1 and pin 3 jumped. No magic smoke from this setup. \$\endgroup\$ – Jack Sep 22 '19 at 4:04

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